Although sliding doors are commonly used in some styles of motor vehicles such as passenger and cargo vans and trucks, their construction and mode of operation are wholly unsuited for use in pressurized aircraft. It is therefore not surprising that heretofore doors of this kind have found no application in aircraft of the kind described.
Aircraft design always necessarily involves numerous tradeoffs in strength, safety, and weight. Those same factors are, of course, involved in the design of an aircraft door. Because of the large pressure difference existing between the exterior and interior of an aircraft intended for operation at altitudes above 35,000 feet, the total force acting on an aircraft door is enormous. The total force acting upon a cargo door with an area which normally is much greater than a passenger door presents an even greater problem.
The design of the door and the related fuselage must take into account the transfer of the force to the fuselage which, simply described, comprises principally a plurality of longitudinally spaced bulkheads of aluminum or other light-weight metal and quite thin aluminum outer skin. When a conventional hinged aircraft door is opened, the weight of the door is carried by a plurality of spaced hinges to the fuselage. Although the door is then not subjected to large air pressure differential on its opposite sides, the door nevertheless exerts a very large turning moment on the supporting hinges.
These unique design considerations inherent in the design of a pressurized aircraft are, of course, entirely different from those involved in the design of, for example, a panel truck or van. Such vehicles are not subjected to the enormous pressure differences that exist only at high altitude. Also, the weight factor is not nearly as crucial in such vehicles as it is in an aircraft and therefore such a door can readily be made as strong as may be required.
The conventional hinged aircraft doors now universally used have frequently presented safety problems. Several instances have been documented in which such a door has inadvertently become opened at high altitude with catastrophic results. Another disadvantage associated with the common hinged door has been in connection with modification of a passenger aircraft to adapt it for service in hauling cargo where the installation of large cargo doors in substitution for the normal passenger doors has proved to be not only very costly but to involve many months of down time for the aircraft which is, of course, a large contributing cost factor in any such conversion.
The aircraft door of the present invention overcomes the drawbacks described above to a remarkable degree. Using techniques quite different from those used in automotive vehicles such as vans and the like, the aircraft door of this invention, as will be further described herein, is inherently safe, readily transfers the pressure forces from the door to the aircraft's fuselage, and can be installed with significantly less labor costs, and much less downtime when retrofitting an existing aircraft for cargo hauling.